1. Field
This invention pertains in general to radiation filters/reflectors and, in one particular embodiment, more specifically to a variable neutron reflector for a nuclear reactor core.
2. Related Art
In a nuclear reactor for power generation, such as a pressurized water reactor, heat is generated by fission of a nuclear fuel such as enriched uranium, and transferred to a coolant flowing through a reactor core. The core contains elongated nuclear fuel rods mounted in proximity to one another in a fuel assembly structure, through and over which the coolant flows. The fuel rods are spaced from one another in co-extensive parallel arrays. Some of the neutrons and other atomic particles released during nuclear decay of the fuel atoms in a given fuel rod pass through the spaces between fuel rods and impinge on fissile material in adjacent fuel rods, contributing to the nuclear reaction and to the heat generated by the core.
Movable control rods are dispersed throughout the nuclear core to enable control of the overall rate of the fission reaction, by absorbing a portion of the neutrons, which otherwise would contribute to the fission reaction. The control rods generally comprise elongated rods of neutron absorbing material and fit into longitudinal openings or guide thimbles in the fuel assemblies running parallel to and between the fuel rods. Inserting a control rod further into the core causes more neutrons to be absorbed without contributing to fission in an adjacent fuel rod; and retracting the control rods reduces the extent of neutron absorption and increases the rate of the nuclear reaction and the power output of the core.
FIG. 1 shows a simplified conventional nuclear reactor primary system, including a generally cylindrical pressure vessel 10 having a closure head 12 enclosing a nuclear core 14 that supports the fuel rods containing the fissile material. A liquid coolant, such as water or borated water, is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18 typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown) such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16 completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20.
Commercial power plants employing this design are typically on the order of 1,100 megawatts or more. More recently, Westinghouse Electric Company LLC has proposed a small modular reactor in the 200 megawatt class. The small modular reactor is an integral pressurized water reactor with all primary loop components located inside the reactor vessel. The reactor vessel is surrounded by a compact high pressure containment. Due to both limited space within the containment and the low cost requirement for integral pressurized light water reactors, the overall number of auxiliary systems needs to be minimized without compromising safety or functionality. For that reason, it is desirable to maintain most of the components in fluid communication with the primary loop of the reactor system within the compact, high pressure containment.
Typical control rod drive mechanism used in existing and proposed small modular reactors require moving parts to be positioned in locations where mechanical and electro-mechanical failure of the mechanism represents a serious operating concern. It is very difficult or impractical to repair failures associated with these control mechanisms and associated position indication sensors while the reactor is operating.
Conventionally, some reactors have employed dense materials to surround a reactor core with the dense materials having a high potential to produce large angle scattering collisions with escaping neutrons, and a low absorption potential, to minimize the neutron leakage from the reactor. This type of material is said to “reflect” escaping neutrons back into the reactor where they can contribute to additional fission reactions. The result is that less reactivity or fissionable material is needed to create a critical reactor configuration. Given a critical reactor configuration with no reflector, the addition of a reflector allows the fission reaction rate in the core to increase, thus producing a higher power level.
It is an object of this invention to replace the functionality of some or all of the control rod drive mechanisms with moving parts with a system with no moving parts.
It is a further object of this invention to provide such a system with no moving parts that is wholly contained outside the core.